It's actually my DSS, who stole it from his DM. DP (and I, to a certain extent) has always been responsible for dealing with this kind of thing but to be honest we're at a bit of a loss.
He's admitted to taking the money from a pot at his mothers house and said he took it to buy a toy he wanted. But he won't explain why he gave it to the other child.
There have been a few other instances of him taking money in the past but usually coins, which he's said he 'found'. But nothing like this.
What would you do?